This invention relates to voice processors and in particular to peak limiting voice processors for single sideband radio communications.
In radio communication systems it is desirable to maximize the use of transmitter power for the broadcasting of information. This is especially true in the area of Single Sideband (SSB) transmissions where the power output of most SSB transmitters is peak power limited rather than average power limited, while the broadcast information (intelligibility) in noise and interference is determined primarily by the average power of the transmitter. Therefore, it is desirable to maximize the average power of an SSB transmitter. More average power can be transmitted with an SSB signal if the peak-to-average power ratio of the SSB signal is reduced from a typical 17 dB peak-to-average power ratio of an unprocessed single sideband voice (audio) signal. This reduction facilitates more average power being transmitted, and thus improves the intelligibility and signal-to-noise ratio of the received signal.
In the prior art, there are three major techniques used to reduce the peak-to-average power ratio of single sideband transmitters. One technique is the use of audio volume compression which reduces the peak-to-average power ratio of the single sideband transmitter by typically 1 dB. A more significant reduction in the SSB's peak-to-average power ratio is achievable by symmetrical clipping of the positive and negative voltage peaks of either the audio waveform (signal) prior to SSB modulation (audio premodulation clipping) or the IF waveform (signal) after SSB modulation (SSB peak clipping). SSB peak clipping has been preferred over audio premodulation clipping. Many of the clipping distortion products resulting from SSB peak clipping fall outside of the IF passband, while the corresponding distortion products in audio premodulation clipping fall back into the desired audio passband. As a result, the inband distortion generated for a given amount of clipping is less for SSB peak clipping than for audio premodulation clipping. Additionally, the SSB waveform or signal is not related to the audio input waveform or signal in a simple manner and SSB peak clipping of the audio signal does not guarantee a reduction of SSB's peak-to-average power ratio under all conditions. Voice symmetrical audio premodulation clipping of 18 dB typically reduces the SSB's peak-to-average power ratio by 4 dB. In contrast, 18 dB of symmetrical SSB peak clipping typically reduces the SSB's peak-to-average power ratio by 8 dB and produces less inband distortion.
Symmetrical peak clipping of the modulated SSB waveform (signal) is generally preferred to clipping of the audio waveform prior to modulation. There are, however, disadvantages to peak clipping of the SSB signal. These include the requirements for a post clipping Intermediate Frequency (IF) or Radio Frequency (RF) filtering to remove spectrum splatter caused by distortion products. Spectrum splatter is defined as a broading over frequency of the concentration of the transmitted energy. An IF or RF filter is more costly than the audio filter used to remove out-of-band spectrum splatter in audio premodulation clipping. It is generally simpler and more economical to clip at audio frequencies than at intermediate frequencies or radio frequencies.
One of the prior art circuits that avoids some of the above enumerated problems provides for generating an SSB signal at IF frequencies, peak clipping this signal, filtering it, and converting it back to audio frequencies. The converted signal is then used to modulate an SSB transmitter. The resulting SSB envelope is limited as if SSB peak clipping had been used in the transmitter. This embodiment requires the additional cost and circuitry of a separate SSB modulator. In addition, care must be taken to prevent the SSB IF signal from leaking into the transmitter and interfering (mixing) with the SSB transmissions.